The present invention relates to connections between layers of a printed wiring board, and more particularly to the use of laser ablation in making such connections.
Connections between layers of a multi-layer printed wiring board are typically created by drilling holes through the dielectric layers so that the drilled holes intersect conductive traces on those layers to be connected, and then applying a metallic layer in the drilled holes, as by “seeding” or electrolessly plating the interior surfaces of the holes. In most cases, the drilled holes are through holes extending through all of the layers of the board, rather than blind vias. This has become a rather common practice because of the difficulties in controlling the depth of a drill used to create blind vias in the materials. For example, in rigid-flexible printed wiring boards where the rigid layers are constructed of materials such as glass reinforced epoxy and the flexible sections are constructed of thin layers of a dielectric such as polymide, it would be difficult to drill and stop at an inner metal layer in the flexible layer. Thus, through hole connections are commonly utilized.
However, through holes are rather large and occupy more board area on both sides, as well as on all intermediate layers of the assembly. This tends to reduce the achievable electronic density and tends to makes the overall package size larger. Clearly, one solution to this problem would be to make use of blind vias. Therefore, there exists a need for an improved method of forming blind vias, and particularly for forming blind vias as connections between rigid sections and flexible layers in rigid-flexible printed wiring boards.